Alkaline fuel cells have been known, at least in rudimentary form, since shortly after the turn of the 20th century. Indeed, alkaline fuel cells have found at least limited success and acceptance because of their use by NASA, particularly since the Apollo missions. Alkaline fuel cells were also used by NASA for the space shuttle Orbiter vehicles. However, there has been much greater commercialization of Proton Electrode Membrane (PEM) fuel cells for a variety of reasons that need not be discussed in detail here.
On the other hand, the market is once again turning to alkaline fuel cells because of several specific advantages that they have over PEM fuel cells. Those advantages include the fact that alkaline fuel cells can be manufactured without having to rely on precious or noble metal electrodes; and that the electrolyte is alkaline and not acidic, which leads to better electrochemical performance and generally broader operating temperatures than those of PEM fuel cells.
The general structure of alkaline fuel cells is quite simple. Typically, fluid channels are formed through the plastic electrode frames for the distribution of gas and electrolyte. Typically, the fuel gas is hydrogen, although it may also be such as methanol vapour, the oxidizer gas is oxygen or air, and the electrolyte is alkaline solution such as aqueous potassium hydroxide solution. One purpose of any design of electrode frames for alkaline fuel cells is to provide for even distribution of the flow of gases across the faces of the electrodes. However, the prior art alkaline fuel cells have had problems relating to the elimination of droplets of moisture which develop in the gas path. Prior art alkaline fuel cells also have had difficulty with respect to thermal stresses that may be caused by uneven currents, typically because of uneven gas flow, among other contributing factors. The present invention seeks to overcome those and other shortcomings of prior art alkaline fuel cells by providing for even distribution of the flow of gases across the face of the electrodes, and by providing design features which effectively eliminate unwanted buildup of droplets of condensate which may be contaminated with electrolyte running down the face of the electrodes.
The present invention also provides designs which reduce thermal stresses that may be caused by uneven currents as they flow through the electrode structures, and which are also caused by thermal cycling. That feature is particularly accomplished by the provision of a metal contact frame embedded in the plastic electrode frame so as not only to improve current collection in monopolar cells, but also so as to significantly reduce the thermal expansion of the plastic frame. This reduces stresses imposed on the electrode as well as stresses imposed on the inter-cell seal, and thereby contributes to improved tolerance of thermal cycling. This, in turn, provides for increased longevity of the stacked alkaline fuel cell structure.
It will be understood by those skilled in the art that the features of the present invention as they are described thereafter may be equally applicable to monopolar cell designs and, with appropriate amendments and alterations as may be required, to bipolar cell designs. Those terms are meant, in this case, particularly to describe stacked alkaline fuel cell structures where monopolar cell structures employ edge current collection, and bipolar cell structures where bipolar plates may be employed for cell interconnects.
The typical material from which plastic frames for flat electrode structures for use in alkaline fuel cells are manufactured is beyond the scope of the present invention, except as will be described hereafter with respect to the stiffness, modulus of elasticity, and coefficient of thermal expansion, of that material. Suffice it to say that such material may be either a thermoplastic material or a thermosetting material. In general, openings are formed through the thickness of the plastic frames so as to provide for passages which permit gas flow or electrolyte flow from one end of the stack structure to the other. A stacked alkaline electric fuel cell structure is assembled by placing flat electrode structures adjacent one to another, observing polarity of the electrodes being put into place, and securing them by such as adhesive, compression, welding and other well-known methods. Accordingly, such a stacked structure with openings in the plastic frames is said to have internal manifolding, as opposed to external manifolding, so that inlet and outlet conduits for gas and electrolyte can be connected to the entire stack structure.
In the design of alkaline fuel cells which employ a circulating electrolyte, the electrolyte enters each cell of the stack at the bottom thereof, and flows upwardly. Exit channels formed at the top of the cell in the frame structure therefore are typically designed so as to permit easy exit of any entrained gas bubbles there may be in the liquid electrolyte. Moreover, as a consequence of the electrochemical reaction which occurs within the fuel cell, water is created in the cell, and as a result condensation will typically form in and outside the electrolyte diffusion layer of any of the electrode structures. This, in turn, may lead to partial wetting and electrode “weeping”, whereby droplets of condensate will contaminate the electrolyte as it runs across the gas face of the electrode. Regrettably, in some extreme cases, it is possible that electrolyte may find a path through imperfect electrode-to-frame seals, or cracks on the electrode surface. This, in turn, may lead to electrolyte leaks.
Any liquid which finds its way into gas spaces of the cells must be promptly removed in order to assure good access of the gas to the working surface of the electrode. This has typically meant in prior art alkaline fuel cells that the gas would flow from top to bottom of each of the individual cells, so as to carry the liquid out of the cell in a manner which provides for the least hydraulic resistance to the flow of fluid, namely downwardly with the assistance of gravity. A typical prior art cell structure provided for flat, thin gas spaces in the individual cells, having one or a plurality of exit slits at the bottom of the cell. However, the problem has been that such bottom slits may become blocked by drops of liquid which remain in place as a consequence of capillary forces. If there is a plurality of slits, and some of them become blocked, then there will be an uneven distribution of gas flow across the face of the electrode, resulting in weakened performance of that cell. If the main exit slit becomes blocked, then the entire cell will malfunction.
Moreover, typical prior art stacked alkaline fuel cell structures relied on parallel feed of gases, where the pressure differential between the inlet and outlet across any cell could be too small to overcome the capillary forces and to blow out the offending drop of liquid. If any one or more individual cells became blocked, such blockage might not be well noticed in the hydraulic behaviour of the stack, even though the electric behaviour may be compromised. This has led designers to arrive at somewhat complicated solutions in which groups of cells are cascaded so as to achieve high flow rates and high pressure differentials. In turn, this requires additional pumping power or, when a blocked cell can be electrically detected, increased gas flow and higher pressure for a short period of time so as to blow out the offending liquid by force.